26F 36M. My girlfriend over reacts and loses her temper at service staff who often have done nothing wrong

Just had an argument with my gf. She left some cosmetics in a hotel, when she emailed the hotel if they had been found the hotel politely replied they were no longer in the room and apologised.

Her response was to accuse the house keeping staff of theft in a very mean and & unprofessional email reply. Essentially saying it's the worst hotel she's ever stayed full of unprofessional theives.

The argument was because I suggested to her not to accuse their staff of theft without proof, and to keep her language professional. I'm ok with her expressing disappointment.

This is not the first time I've seen this, especially to hotel and restaurant staff with often reasonable requests are met with dispositionate hostility.

My feedback that she's overreacting and behaviour is not appropriate is met with “I can't believe you're taking their side”.

This is really making vacations / meals uncomfortable for me as it seems to happen most trips and she often expects me to back her up in her abusive behavior.

Any tips on taming this temper? Is this a red flag for how she might be with me in the future once we settle?